A Mark IV Amphibian is an early model of British

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 naval oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 rebreather

Rebreather

A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...

 made by Siebe Gorman

Siebe Gorman

Siebe Gorman & Company Ltd was a British company which developed diving equipment and breathing equipment and worked on commercial diving and marine salvage projects...

. It was arranged like a UBA, but its oxygen cylinder is smaller. It was so called because it could be used for diving

Diving

Diving is the sport of jumping or falling into water from a platform or springboard, sometimes while performing acrobatics. Diving is an internationally-recognized sport that is part of the Olympic Games. In addition, unstructured and non-competitive diving is a recreational pastime.Diving is one...

, or as an industrial breathing set on land.

The Mark IV Amphibian is sometimes called the 'I'm an Octopus' Suit, due to the popular saying "I'm an Octopus and I like Divers".
The Mark IV Amphibian is also currently being reproduced in a factory in Hong Kong called Walsh's facilities (English Translation) where they are being distributed to museums around Europe.